I voted Greek. For me, Greek myth is extremely involved and there are many, many stories you can spring off from. Granted Celtic mythology is also very involved, but it's never been quite up my alley. If you were to say Viking myth, now that I could really sink my teeth into!
My own personal preference is Egyptian though. There aren't nearly enough Egyptian stories out there that aren't crowded with walking dead guys, mummies.

So I set out to do something about it.
Myth, folklore, and legends all have a bit of truth to them. One may even say that the gods of old were merely men who were raised to god-like status because of their deeds or misdeeds. There isn't much of a distinction, in my opinion.
HP, don't even worry yourself. Your story will still be original. To be honest, I haven't read any of the HP books but I have watched the movies. Something just won't let me sit through a book as long as the HP ones where everything is centered around young adults. I prefer more adult geared stories where a little bedroom action can come at me at any moment. Even funnier, I won't read Romance novels either. Why? Because you expect the plot to get good, but usually it's just a BS plot with BS characters geared toward women who don't have anything better to do with their time. I, for one, am a woman who loves adventure, action, a touch of romance, and lots of complicated plot lines. To me, that's better than who did who, what your ex did to your sister, and the other crap you read about in Romance novels. I guess the whole reason I don't do romances is because I've got a romantic husband and don't need to be filled up on someone else's torrid love affairs.
Sorry for the off-subject babble.
Lynn